Even as federal health officials announced plans to speed development of an Ebola vaccine, scientists who study the virus warned that the task would be arduous and that success is hardly guaranteed.

The Ebola virus is “a survivor,” said John Dye, chief of the viral immunology branch at the U.S. Army Medical Research Institute of Infectious Diseases. “It does what it can to avoid the human immune system.”

The government plans to fast-track development of a vaccine shown to protect macaque monkeys, aiming to test it in humans as early as next month.

If the vaccine proves effective, it may be given to health care workers and others at high risk for infection sometime in 2015, said Dr. Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases.

But the development effort depends on several contingencies: fast regulatory approval of the trial, the first of its type in healthy humans; results proving the vaccine is safe and provokes an immune response; and, perhaps most critically, attracting the interest and investment dollars of the pharmaceutical industry.

Assuming the necessary Food and Drug Administration approvals are obtained, Fauci said, about 20 volunteers will receive the experimental vaccine beginning in mid-September.

“You’ve got to be really careful,” he said. “This is the first time this will be put in a human.”

Some scientists have called the fast-track approach aggressive and optimistic, warning that human trials inevitably encounter stumbling blocks, including adverse reactions, and that it often takes many recalibrations to figure out the optimal dosages.

Ebola is strangely elusive, raging through populations like wildfire before disappearing. Unlike viruses that attack only the respiratory system or only the gastrointestinal system, Ebola attacks almost every cell in the human body.

But it does not persist long-term in the bodies of the infected, as does HIV.

Since it was identified nearly 40 years ago, Ebola has infected fewer than 5,000 and killed fewer than 3,000 in Africa, home to 1.1 billion people. By contrast, more than 35 million people worldwide are infected with HIV, and close to 39 million have died of its complications.

The unpredictable and sporadic nature of this infection has made an Ebola vaccine an unattractive goal for drugmakers.

“It’s not really a good target for a pharmaceutical company,” said Kartik Chandran, an associate professor of microbiology and immunology at the Albert Einstein College of Medicine in New York who researches Ebola. “It’s not clear you could even earn back your investment.”

The vaccine to be tested in humans relies on a benign virus that carries two proteins from the surface of the Ebola virus. The proteins help the virus penetrate human cells.

If the vaccine is successful, the immune system will be trained to recognize the proteins and to mount a strong response should it encounter the virus.

The initial human trial is intended to evaluate the safety of the vaccine and to see whether it induces an immune response similar to the one produced in the monkeys.

The immunized animals were deliberately exposed to the Ebola virus and survived. But human volunteers will not be exposed to the virus, Fauci said.

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